I'm sorry, this looks like self-defense to me (and I'm about as pacifist as they come). It doesn't matter that the keeper has knocked the attacker to the ground- until the attacker is disabled, or others come to restrain the attacker, he is still a threat. You don't give a wacko a second chance to come at you again.
If a person attacks you on the street out of nowhere with a flying karate kick, and you manage to knock them to the ground, you either run like hell, or you continue to subdue the threat (flight or fight). It's instinctive survival. If the attacker were a wild animal or a pitbull (instead of an agro wacko), do you think you wouldn't keep hitting it to keep it on the ground and subdued? Damn right you would, until it turned tail and ran away. Watch the wacko- he doesn't try to escape. He is not subdued. As far as the keeper is concerned, the wacko isn't finished yet.
You can see, once the keeper backs off after security has secured the attacker, he is still on his guard as he senses another person running towards him. Once he realizes it's not another wacko, he finally senses the threat is over and relaxes his guard.
The wacko should be banned? Hell, that's assault, he's going to prison. This is no different than the nutjob who stuck a knife in Monica Seles during a match. Too bad she didn't see it coming- if she had, you know damn well she would have smashed him repeatedly in the face with the metal edge of her racquet until help arrived.